1. Field of the Invention
The present invention relates generally to the field of signal transmission and more particularly to a signal transmission apparatus adapted for use in conjunction with a nonlinear transmission device.
2. Description of the Related Art
In general, a nonlinear transmission device, such as a high power amplifier (HPA), amplifies an up-converted radio frequency signal and transmits the amplified signal to a receiving end. Since the HPA is generally implemented as an active device having nonlinear characteristics, an output signal inevitably includes a distortion component. Various linearizing techniques and algorithms, including feed forward, predistortion, envelope correction and bias compensation, have been introduced to improve the nonlinear characteristics of such active devices.
FIG. 1 illustrates a conventional signal transmission apparatus adapted to use predistortion and look-up table (LUT) linearization techniques. The apparatus includes a nonlinear transmission device 15, a single LUT 10 for storing a plurality of gain values for improving the nonlinear characteristics of nonlinear transmission device 15, an indexer 11 for addressing a gain value of LUT 10 according to the magnitude of an input signal (Vi[n]), a complex multiplier 12 which produces a predistorted output Vd[n] after processing input signal Vi[n] and the gain value outputted from LUT 10. Predistorted output Vd[n] is fed to nonlinear transmission device 15 via a digital-analog-converter (DAC) 13 and an up frequency converter (UR) 14.
A signal capture module (SCM) 18 stores predistorted output Vd[n] and the output (Vf[n]) from nonlinear transmission device 15. A digital signal processor (DSP) 19 is coupled between SCM 18 and LUT 10 and is used to update LUT 10. Nonlinear transmission device 15 is typically a HPA with complex multiplier 12 serving as a predistorting device. A down frequency converter (DC) 16 and an analog-to-digital converter (ADC) 17 are coupled between the output terminal of nonlinear transmission device 15 and SCM 18, as shown in FIG. 1.
LUT 10 generally includes N number of entries addressed by indexer 11 whereby each entry stores complex gain values. Indexer 11 calculates the magnitude of input signal Vi[n], addresses an entry of LUT 10 according to the calculated magnitude, and outputs a specific complex gain value (referred to as ‘gain value’, hereinafter).
Predistorter 12 multiplies input signal Vi[n] and the gain value output of LUT 10 to generate predistorted output signal Vd[n]. Nonlinear transmission device 15 amplifies the inputted Vd[n] to a predetermined level generating output signal Vf[n]. SCM 18 stores Vd[n] and Vf[n], as shown in FIG. 1. If DAC 13, UC 14, DC 16 and ADC 17 were to operate ideally, Vd[n] and (Vf[n]) would exhibit input/output characteristics of nonlinear transmission device 15, respectively. DSP 19 estimates the nonlinear characteristics of nonlinear transmission device 15 on the basis of inputted Vd[n] and Vf[n] and updates LUT 10 to compensate for the same accordingly. If DSP 19 repeatedly updates LUT 10, compensation values stored in each entry of LUT 10 would have completely reverse characteristics relative to the estimated nonlinear characteristics of nonlinear transmission device 15.
Thereafter, when a gain value stored in LUT 10 according to indexer 11 is outputted to predistorter 12 and multiplied with input signal Vi[n], the predistorted output signal Vd[n] would exhibit completely reverse characteristics relative to the nonlinear characteristics of nonlinear transmission device 15. Nonlinear transmission device 15 amplifies Vd[n] whereby its output Vf[n] does not include a distortion component, i.e., the nonlinearity of nonlinear transmission device 15 has been effectively compensated.
FIG. 2 is a graph showing conventional operational characteristics of nonlinear transmission device 15 according to operation area. Operation areas (of nonlinear transmission device 15) may be classified, for example, by operational power (P) level. Nonlinear transmission device 15 exhibits different nonlinear characteristics according to individual operational power levels (P1, P2, . . . , PM) discriminating the various operation areas, as shown in FIG. 2.
If the operational characteristics of nonlinear transmission device 10 were linearized, performance would be degraded whenever the operation area changes. Such performance degradation may be resolved by updating the LUT according to the change in characteristics of nonlinear transmission device 10. However, performance degradation occurring during updating of the LUT cannot be avoided. Specifically, whenever the operation area of the nonlinear transmission device is quickly changed, performance is severely degraded.